fix(electron-updater): AppImage install no longer deletes the running app before the new one is in place - #10386

Problem

AppImageUpdater.doInstall deletes the running AppImage first (unlinkSync(appImageFile)) and only then runs execFileSync("mv", ["-f", installerPath, destination]), which resolves mv through PATH. If mv fails, the old AppImage is already gone and the user has no app to launch.

Real-world report: pingdotgg/t3code#23188. mv failed with spawnSync mv ENOTCONN because the dead FUSE mount dir ($TMPDIR/.mount_*) of an earlier AppImage launch was on PATH. The next install attempt then failed with ENOENT on the unlink, and the app could not be launched (exit 127).

Related: #8698 reports the same failure: the old AppImage was removed, then mv failed to move the download.

Fix

• All validation (APPIMAGE checks, missing installerPath) now runs before any file is touched. The unlink-first step is gone.
• The destination rule is unchanged (Autoupdate AppImage renames file #2964: a versioned file name follows the download, a custom name is kept).
• The new AppImage is staged as .<old name>.update next to the destination (linkSync, falling back to copyFileSync on EXDEV or a filesystem without hard links), then renameSyncd over the destination. That swap is atomic and runs no external command. On any error the staged file is removed and the error is rethrown, so the old AppImage stays in place.
• After a successful swap the downloaded file is removed (as mv did). When the file name changed, the old AppImage is removed and appimage-filename-updated is emitted, as before.
• The relaunch logic is unchanged.

Verification

Two new tests in test/src/updater/baseUpdaterUnitTest.ts (AppImageUpdater doInstall file replacement):

1. Install succeeds with PATH pointed at an empty dir, and the AppImage holds the new content.
2. When the swap fails (no downloaded file), the old AppImage is unchanged.

On master (without the fix), both fail:

× replaces the AppImage without a PATH lookup
Error: spawnSync mv ENOENT

× keeps the installed AppImage when the swap fails
Error: ENOENT: no such file or directory, open '.../TestApp.AppImage'

With the fix:

• TEST_FILES=baseUpdaterUnitTest,linuxUpdaterUnitTest pnpm ci:test: 2 files, 67 tests passed
• pnpm compile && pnpm ci:validate: passed
• pnpm format:check: passed

A changeset (electron-updater: patch) is included.

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